Rayleigh mode in amorphous hydrogenated carbon films

Brillouin-scattering experiments are performed on amorphous hydrogenated carbon (a-C:H) films grown by plasma deposition on silicon substrates at various bias voltages (-50 to -1400 V) and annealed after deposition at a bias of -400 V between 100 and 1100 °C. Scattering from Rayleigh surface phonons is observed. The results show that the Reyleigh mode frequencies are between those of diamond and graphite. With increasing substrate bias voltage the Rayleigh mode frequency first increases to a maximum value at -200 V and then decreases monotonically. Upon annealing, the Rayleigh phonon frequency decreases drastically for temperatures above 500 °C with minor changes below 400 °C. These results correspond to the results of the ${\mathit{sp}}^3$-to-${\mathit{sp}}^2$ bonding ratio, indicating that the frequency variation of the Rayleigh phonon is due to phase transformations of a-C:H. The coupling between the relaxation mechanism and the phonon also confirms the phase transitions. Based on the hydrogen concentration, the ${\mathit{sp}}^3$-to-${\mathit{sp}}^2$ bonding ratio, and the phonon frequency, the correlated densities and elastic constants of annealed a-C:H films are calculated.